The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services from geosynchronous orbit. Market demands for such spacecraft have imposed increasingly stringent requirements on spacecraft payloads. For example, broadband service providers desire spacecraft with payloads offering increased data rate capacity at higher effective isotropic radiated power (EIRP) through each of an increased number of user spot beans.
A multi-beam antenna (MBA) system generates a set of user spot beams that define a coverage area which may extend, in aggregate, across a large region on the ground. MBA's providing wide-band communications services from a geosynchronous satellite conventionally provide contiguous coverage of a region with a triangular lattice of overlapping circular antenna beams. These beams are conventionally formed using close packed clusters of circular feed horns, also centered on a triangular lattice. The feed horns illuminate, for example, offset fed parabolic reflectors to provide the desired antenna gain. Since, in order to provide contiguous coverage, the circular beams must overlap, the unique coverage area of each individual beam is normally defined as the inscribed hexagon of the edge of coverage circular gain contour, as illustrated in FIG. 1. The corners of the hexagon are located at the points where the edge of coverage gain contours from three adjacent beams cross. The sides of the hexagons represent points of approximately equal performance between adjacent beams.
An objective of many MBA systems is to enhance spectrum utilization efficiency by providing for frequency reuse among the multiple antenna beams. To avoid interference near the beam edges, known payload designs provide that unencoded signals in adjacent, and nearly adjacent, beams utilize distinctly different combinations of frequency sub band and polarization. For example, a “four-color” frequency reuse scheme, also illustrated in FIG. 1, is known, where each color of the four color scheme defines a particular combination of frequency sub band and polarization. Features of such a coverage pattern are discussed in Gehring, et al., “Trade-off for Overlapping Feed Array Configurations, 29th ESA Antenna Workshop on Multiple Beams and Reconfigurable Antennas”, April 2007 (hereinafter “Gehring”), the disclosure of which is hereby incorporated by reference. Interference mitigation may also be achieved by various encoding schemes, as disclosed by Cottatellucci, et al., “Interference mitigation techniques for broadband satellite system”, ICSSC 2006, 24th AIAA International Communications Satellite Systems Conference, 11-15 Jun. 2006, San Diego, USA (hereinafter, “Cottatellucci”), the disclosure of which is hereby incorporated by reference.
Another objective of an MBA system is to maximize beam forming efficiency, measured as gain area product (GAP) of the MBA divided by 4π steradians (41,253 square degrees). GAP=Gmin*Acov, where Gmin is the gain over coverage area, Acov, with Acov expressed in square degrees. Known MBA systems provide a GAP of 10000-16000 and, therefore, a beam forming efficiency in the range of 24% to 39%, although higher efficiencies are desirable See: Han, C. C., et al., “Satellite Antennas”, Antenna Handbook, volume 3, chapter 21, edited by Lo, Y. T., et al., ISBN 0-442-01594-1 (hereinafter, “Han”), the disclosure of which is hereby incorporated by reference.
As the above mentioned references, at least, make clear, there is a long-felt need for a high throughput spacecraft that more efficiently uses available frequency spectrum and improves beam forming efficiency of a multi beam antenna.